Preparation Of Nitrogen-doped Carbon Materials And Their Electrochemical Performance For Supercapacitor

Supercapacitors, as a new type of energy storage device, have the superior properities of high power density, long cycling life, and wide range of temperature adaptation, which have broad application prospects in the area of transport, energy, electronic information, and so on. Electrode materials are one of the most important parts in supercapacitors, whose composition and structure can affect the overall performance and the application fields of the supercapacitors. Carbon materials are the most common electrode materials used for supercapacitors, which is one of the hot research directions in the field of basic research. In a sense, it is essential to understand and be awareness of carbon in the development of supercapacitors. The main indexes of excellent electrode materials to evaluate the performance of supercapacitors are high specific surface area, suitable pore structure, surface chemical properties, good electrical conductivity, and high cost-effective. Therefore, many researches are focused on the optimization of pore structure and the surface modification of carbon materials, intending to improve the specific capacitance of the capacitor and to expand working window, which can improve the overall performance and increase the energy density of the capacitor. In this paper, polymer materials, melamine, and carbon dioxide were used as the raw materials, the nitrogen precursor and the active reagent for physical activation, respectively. The structures of samples were characterized by N2adsorption, elemental analysis, XPS and XRD. The electrochemical properties were measured by three-electrode system and through assembling capacitors to test the performance at high working voltage. The main results are shown as follows:(1) The nitrogen doped activated carbons (NACs) were prepared by carbonization and activation of the mixture of melamine and PET polymer plastic in CO2atmosphere. With the increase of activation time, the specific surface area and total pore volume increase, but the bulk nitrogen content decrease while the surface nitrogen content firstly reduce then small amplitude recovery. NAC-4has the specific surface area of1103m2g-1, total pore volume of0.53cm3g-1, and the surface nitrogen content of4.92wt.%。What is more, the specific capacitance of NAC-4is226F g-1in6mol L’1KOH while260F g-1in1mol L-1H2SO4at the current density of50mA g-1. In addition, two-electrode test showed that the capacitor based on symmetric NAC-4can work at the high voltage of1.8V in the neutral electrolyte of1mol L-1Li2SO4, which is much larger than in acid and alkali electrolyte and shows a better cycle stability.(2) Various processes (carbonization-activation, carbonization-activation-carbonization) were taken from the nitrogen-rich precursor composed of melamine and low temperature treatment of PET to prepare the NACs. The effects of carbonization temperature of nitrogen-rich precursor and the processes were investigated. Results showed that the preparation processes and condition can affect the surface chemistry composition of NACs and be implemented in the modulation of nitrogen and oxygen species on the carbon surface. The nitrogen precursor carbonized at the temperature of400℃possesses more content of nitrogen and oxygen than at600℃. The specific surface area and total pore volume increase significantly after activation. The sample carbonized at400℃has the specific surface area of815m2g-1and the nitrogen content of3.0at.%, which shows the highest specific capacitance of191F g-1at the current density of50mA g-1and still remains87%until the current density reaches1000mA g-1, indicating the good capacity property.(3) The nitrogen doped carbon materials (NCPs) were prepared by using the polymer resin as raw materials with the same preparation procedure of PET. The sample NCP-3has the nitrogen content of2.0wt.%and shows good capacitance characteristics, attribute to its low internal resistance and short response time. Asymmetric capacitor NCP/AC was assembled by the electrode of NCP-3and commercial AC for the positive and negative, respectively. Electrochemical test presented that NCP/AC shows the lower resistance and larger energy density than symmetric capacitor NCP/NCP at high working voltage. What is more, NCP/AC presents good cycle stability under different working voltage.